Current engine designs such as those used in automotive and many other applications typically use an in-line or “V” arrangement of multiple pistons. Cams on one or more camshafts are used to open and close each valve. These camshafts are aligned parallel to the crankshaft that controls the oscillatory motion of the pistons. They extend approximately the entire length of the engine. The possible locations of the camshafts are limited due to the need to arrange them in concert with the valves they operate.
The camshaft is sometimes located above the cylinder head (overhead cam) in close proximity to the valves it operates. One cam is located on the camshaft for each valve operated by the camshaft. Motion from the cam rotation is transferred to the valve by a mechanical lever known as a “rocker arm.”
The camshaft may also be located in the cylinder block with one cam located on the camshaft for each valve it operates. Motion of the cam is transferred to a location near the valve it operates by a mechanical link called a “push rod.” Motion from the push rod is transferred to the valve by a rocker arm in the same manner as described in the prior paragraph.
In modern engines, proper adjustment of the valve actuation mechanism is maintained by use of a small device called a “valve lifter” and is usually installed in the block or head between the cam and the push rod or valve rocker. This device uses engine oil that is supplied by the engine lubrication system.
In older engines, the camshaft was typically located in the engine block near the crankshaft. In this design, the timing of the valve opening and closing is accomplished by gears, one on the crankshaft (timing gear) and one on the camshaft. Newer engines typically have camshafts installed in the cylinder head (overhead cam), and timing is accomplished by connecting pulleys on the crankshaft and on the camshafts by a timing belt. Timing belts are subject to wear and failure and must be replaced at scheduled intervals. Replacement of a timing belt is a costly and inconvenient maintenance service.